1. Field of the Invention
The present invention is in the field of measuring instruments, and specifically relates to a method for correcting the measurements made by a nondispersive infrared (NDIR) gas sensor for the effects of long-term drift.
2. The Prior Art
Many kinds of sensors experience long-term drift. Depending on the sensor, this drift may result from various causes, including: gradual chemical changes; a slow build-up of foreign matter, such as might result from exposure to water vapor, smoke, dust, etc.; and, variations in the electrical power supply, including battery degradation over time.
The classic approach to this problem is to provide within the sensor a reference channel that specifically measures the degradation caused by the aforementioned factors. However, provision of this reference channel increases the cost and complexity of the sensor.
The problem of uncompensated drift is especially serious in NDIR sensors in general and especially in carbon dioxide sensors that are used in fire detectors and in ventilation monitors, because these devices are expected to operate for long periods of time, perhaps even years, without attention. These uses require long-term stability to avert excessive false alarm rates and erratic ventilation.
Fortunately, under some conditions that are not uncommon, the situation is not hopeless. Using the method and apparatus described below, a sensor can be made to calibrate itself, and to compensate itself for long-term drift.
The method will be illustrated by an actual example in which a carbon dioxide sensor is used to determine the concentration of carbon dioxide gas in a building. The application of the method to the detection of other gases by the use of NDIR gas sensors is straightforward.